An important and relatively recent concept has arisen in the neurosciences regarding the functional organization of neural networks and how such systems generate rhythmic behaviors. The salient feature of this concept is that a neural network has the capacity and plasticity to generate various patterned outputs, depending on the specific chemical and/or synaptic inputs the neural network receives. Importantly, the "conservation-modulation" strategy may also be used by the nervous system for the generation of increasingly complex behaviors expressed at later developmental stages. It is thought that the development of adult-specific locomotor behaviors in vertebrates may be based on such a strategy. The proposed research will examine the developing central nervous system of the holometabolus insect, Manduca sexta, where developmental construction of adult behavior appears to be based on the neuromodulation of neural circuitry present at earlier developmental stages. The proposed research will examine the cellular and developmental aspects of neural elements underlying adult ecdysis, a hormonally activated rhythmic behavior that enables the insect to escape from its old cuticle. Previous experiments indicate that the larval ecdysis motor pattern can be un-masked in the adult. Its retention in the adult is thought to contribute to the novel adult-specific ecdysis pattern. By the influence of descending neural activity, in some unknown way, the conserved larval ecdysis pattern-generating circuitry is acted upon to produce adult ecdysis behavior. Preliminary studies indicate that within a group of approximately 15 pairs of descending thoracic neurons, there are modifying interneurons that are necessary for the transformation of the larval ecdysis motor program into the adult-specific one. I propose to: (1) examine the functional role that individual descending interneurons play in the transformation of the larval ecdysis pattern to that of the adult pattern, and (2) examine the developmental history of descending thoracic interneurons, and the anatomical connections these neurons make with the conserved larval circuitry in the adult. Intra- and extracellular electrophysiological methods will be used to study the actions of the descending interneurons on identified ecdysis out-put motoneurons and possibly other interneurons. Various cell tracers will be used to study the anatomy of the descending thoracic interneurons, especially during metamorphic development. This is the first invertebrate model system developed where it is possible to study, at the cellular level, how adult behavior arises from the apparent modulation of a motor program expressed earlier in development.